1. Field of the Invention
This invention relates to low power receiver protector/clutter automatic gain control circuitry for microwave receiver front ends and more particularly to a high temperature superconducting low power receiver protector/clutter automatic gain control.
2. Description of the Related Art
Receiver protectors act as fast acting sophisticated circuit breakers to prevent damage to sensitive radar receivers. This damage can result from power reflected from the antenna during the transmit period or from nonsynchronous pulses due to high power EMI or high power jamming. The receiver protector should provide certain common characteristics such as low insertion loss, fast recovery period and low leakage power under a wide spectrum of environmental conditions and over a wide range of power level and ambient temperatures. They should also have an operating life of at least several thousand hours with a high degree of reliability.
The clutter automatic gain control (CAGC) within a radar receiver is a variable attenuator used to adjust the clutter signal level to a prescribed level. This prescribed level is typically 6 dB below the saturation level of the analog to digital converter within the radar receiver.
The clutter automatic gain control is utilized to provide a uniform return signal allowing the receiver to monitor a smaller range of signals. Generally, the clutter automatic gain control provides high attenuation immediately following the transmit pulse and subsequently steps the attenuation down in a prescribed manner (e.g., linear ramp of attenuation over time) to monitor targets at a farther distance.
Clutter automatic gain controls are also designed to have a low power receiver protection function (LPRP) against the transmitter power leakage which can be on the order of 10 watts following the conventional high power receiver protection function (HPRP). The power allowed into the low noise amplifier within the radar receiver must not exceed 15 dBm typically. An additional level of receiver protection is usually achieved by turning the clutter automatic gain control to maximum (snuff) attenuation (60 dB typical) during the transmit cycle.
Conventional radar receivers use PIN diodes as variable attenuators. These attenuators have a dynamic range which is limited by an input third order intercept (ITOI) on the order of 30 dBm. In addition, the conventional devices have a low intercept point which may vary with attenuation. It is preferred to provide a high intercept point which remains constant during fluctuations in attenuation to avoid distortion.
The PIN diodes are usually biased within the conventional devices such that the received high level signal will trigger the PIN diode. Such an arrangement provides instantaneous protection by creating a large loss within the low power receiver protector and clutter automatic gain control.
The attenuation within the conventional devices could be controlled by varying the current to the PIN diodes. However, this configuration is not preferred inasmuch as the PIN diodes cause intermodulation when the PIN diodes are not completely on or completely off.
Conventional Receiver protector/Sensitivity Time Controls (RP/STC) in radar systems are usually either waveguide or microstrip devices. The waveguide versions have low insertion loss, less than 1 dB at X-band (10 GHz). However, they may be quite large (e.g., 2".times.2".times.6" at X-band to 8".times.8".times.24" at L-band (1-2 GHz)). Such devices are designed to protect against high power leakage from the transmitter and utilize glass or quartz vials containing gas which becomes an ionized plasma under high power. The gas stages can handle (i.e., reflect) these high power levels, but provide no protection below about 10-20 watts of RF power. The gas stages are followed by several diode stages containing PIN diodes shunted across the waveguide to attenuate the reduced power following the gas stages.
The limiter diodes may also be utilized as a controlled attenuator stage since variable attenuation STC function is desired. The attenuation is set by feeding a controlled current through the diodes. Utilization of the limiter diodes as a STC attenuator reduces the number of diodes required and the overall insertion loss.
The limiter diodes are made with a thin I-region to protect at low power, and the diodes begin to turn on under strong receive signal conditions. This produces undesirable third-order mixing products and cross-modulation which can produce false targets and/or hide desired targets in noise.
Microstrip versions of the RP/STC have also been used in conventional applications. These versions consist entirely of diode stages but have limited power handling capability (e.g., 20 W max). They are used in T/R modules and other applications where the waveguide versions can not be utilized. The insertion loss of the microstrip versions of the RP/STC is about twice that of the waveguide diode stages. A typical STC attenuator is required to provide 45-60 dB attenuation.
Therefore, there is a need for a low power and very low loss receiver protector/clutter automatic gain control for radar receiver with an increased dynamic range and which can be optimized for third-order intercept (high limiting thresholds) without hurting the noise figure of the system.